Clean up useless null check.

I think we do not need null check here:

https://github.com/apache/beam/blob/master/runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/control/FnApiControlClient.java#L151

Because before the the `onNext` call, the `Future` already put into the queue in `handle` method.

I found the test as follows:

 @Test
 public void testUnknownResponseIgnored() throws Exception

I do not know why we need test this case? I think it would be better if we throw the Exception for an UnknownResponse, otherwise, this may hidden a potential bug. 

Please correct me if there anything I misunderstand @kennknowles